Method

1. Remove the skin from the fish and cut the fillets into 3cm cubes.
2. Place in a saucepan with the onion and lemon juice
3. Add the stockand milk.
4. Cover and cook gently for 10 minutes.
5. Drain the fish, saving the stock.
6. Melt the margarine in a different saucepan. Add the flour and cook for 2 minutes until it forms a soft ball.
7. Remove from the heat and add the fish stock gradually, stirring continuously.
8. Stir in the Dijon mustard.
9. Bring to the boil, reduce the heat and cook for another 2 minutes.
10. Grease a pie dish and add a little sauce.
11. Place the fish in the dish and cover with remaining sauce.
12. Spoon the mashed potatoes, covering the fish and the sauce. 
13. Brown under the grill or bake in the oven for 30 minutes or until golden brown.